The present invention relates to a printed-circuit construction in which an erasable programmable read-only memory is mounted on the surface of a printed-circuit board.
EPROM (Erasable Programmable ROM) IC devices are widely used in various digital electronic systems in recent years.
EPROMs can easily be reprogrammed by users since written information can be erased by exposure to ultraviolet radiation and new information can easily be written therein.
The EPROM is composed of floating-gate MOS transistors providing nonvolatile storage of information. The U.S. Pat. No. 3,660,819 discloses a floating-gate MOS transistor in detail.
An EPROM IC and other control and drive IC devices are mounted on a printed-circuit board, and used as components for a digital electronic system. The printed-circuit board can be dismounted from the digital electronic system, so that information stored in the EPROM IC can easily be rewritten.
FIG. 1 of the accompanying drawings illustrates a conventional arrangement in which an EPROM IC device is mounted on a printed-circuit board.
As shown in FIG. 1, an insulating baseboard 2 is formed of glass epoxy resin, for example, and an electrically conductive pattern 1 and contacts 15 are printed on the insulating base board 2.
An EPROM IC device 3, has gate pins 7 inserted in through holes 14 in the baseboard 2 and soldered in place. The EPROM IC device 3 includes an EPROM IC chip 10 housed in a ceramic package 5 having a window 12 for passage of ultraviolet radiation therethrough.
The package 5 is composed of a ceramic base 6 having a chip support 9 in the form of a sintered layer of gold paste, a lead frame having a plurality of lead pins 7, and a ceramic cap 13 having the window 12. The IC chip 10 is thermally pressed and fixed to the chip support 9, and has electrodes connected by thin metal wires 11 to the lead pins 7.
The ceramic base 6, the ceramic cap 13, and the lead frame are bonded by a glass material 8 having a low melting point at a sealing tempearture ranging from, 400.degree. to 500.degree. C.
Since the sealing temperature is high when the ceramic package 5 is employed, aluminum pad electrodes of the IC chip 10 and the lead pins 7 are connected by wires of the same material, that is, aluminum wires. This prevents the connecting wires from being subjected to a resistance increase or breakage which would otherwise be caused by the formation of an alloy of the aluminum pad electrodes and a different material, that is, gold wires.
In order to keep the substrate of the IC chip 10 at an earth potential level, the sintered layer 9 of gold paste and the lead pins 7 are required to be connected by thin metal wires. To prevent the sintered layer 9 and the thin metal wires from forming an alloy, a small silicon piece (ground dice) having an aluminum layer on its surface is fixed to the surface of the sintered layer 9, and the ground lead pin 7 is connected by an aluminum wire to the pad electrode of the IC chip through the surface of the ground dice.
As described above, quite a complex process is required to seal the IC chip in the ceramic package.
The printed-circuit board can be used as a component of the digital electronic system by inserting the contacts 15 on an end of the insulating baseboard 2 into a female connector (not shown).
Thus, the conventional EPROM construction is disadvantageous in that the IC package takes up a large area of the printed-circuit board and it is difficult to reduce the thickness of the entire construction. The mounting cost is increased by placing the package with the IC chip mounted therein on the printed-circuit board.
To solve the above problems, there is known chip-on-board technology in which an IC chip is placed directely on the surface of a printed-circuit board, and the IC chip and the printed circuit are covered with synthetic resin.
It has however been difficult to apply the chip-on-board technology to EPROM IC chips since the EPROM IC chip is required to have a window for receiving ultraviolet radiation therethrough.